| Summary: | NASA’s GRACE and GRACE-FO satellite missions provide reliable, direct measurements of ice-sheet mass balance at seasonal timescales but integrated over length scales too large to resolve individual outlet glacier changes. Satellite altimetric measurements of ice-sheet volume change, such as from NASA’s ICESat-2 mission, provide elevation changes at sufficient resolution to resolve individual glacier systems; however, the along-track sampling occurs throughout a 3-month window and conversion of its volume change estimates to mass change requires knowledge of the evolution of the firn air content. While models of firn air content are available, their accuracy is relatively unknown and their evolution is largely driven by the atmospheric model used to force the firn densification model, which also introduces uncertainty in firn evolution. As a result, estimates of ice-sheet mass balance differ significantly from one another, depending on the type of satellite measurements, the selection of atmospheric forcing, and the densification model used.Rather than using these unique data in isolation, we combine the GRACE-FO mass solutions, ICESat-2 volume changes, surface mass balance and firn models to find a best-fit solution to all three, providing new, constrained surface mass balance and ice-sheet mass balance results for both the Greenland and Antarctic Ice Sheet beginning in 2019. In such a manner, we retain the benefits of each individual measurement, providing mass balance at the fine spatial resolution of ICESat-2 with the mass constraint of GRACE/-FO.
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